The addition of hydrogen cyanide to isophorone has long been know. The reaction takes place in a base-catalyzed manner at an elevated temperature. ##STR1##
Published German patent application DE-AS 10 85 871 discloses a method in which isophorone and hydrogen cyanide are reacted at 125.degree. to 275.degree. C. in the presence of a strongly alkaline catalyst which forms cyanide ions and preferably in the presence of a strong polar solvent such as dimethylformamide or dimethylacetamide. The hydrocyanic acid is added essentially at the speed with which it is reacted. Among the catalysts mentioned are alkali metals and their carbonates, alkali- and alkaline-earth alcoholates, -oxides, -hydroxides and -cyanides, as well as amines and quaternary ammonium bases are named as catalysts. The catalytic concentration is given at 0.1 to 20% by weight relative to the weight of the reaction mixture. After the reaction, the reaction mixture is combined with phosphoric acid in order to neutralize the catalyst and it is subsequently distilled. As is apparent from the relevant example of this patent, the hydrogen cyanide had to be introduced very slowly, which resulted in an unsatisfactory space-time yield. A further disadvantage is the fact that a heterogeneous solvent had to be used, which increased the expense for the workup of the reaction mixture. Isophorone nitrile was obtained in only an average yield and moderate purity.
German patent DE-PS 12 40 854 discloses an improvement of the above-mentioned method in that a higher yield can be achieved in the absence of a solvent with lesser amounts of catalyst, to wit, 10.sup.-1 to 10.sup.-3 % by weight relative to the reaction mixture. However, this improvement did not shorten the reaction time - 4 hours dwell time in the main reactor and 1 hour in each of two post-reactors in accordance with Example 2 of the patent.
The applicant of published Japanese application JP-A 57-116038 attempted to improve the method cited referring to the previously evaluated DE-AS 10 85 871 and DE-PS 12 40 854. According to the comparison tests disclosed in the Japanese document, a yield of 53.7% was obtained using a methanolic NaOH solution as catalyst (0.9 ml 15% NaOH per 204 g isophorone) after a total reaction time of 4.5 hours and a yield of 71.1% was obtained using K.sub.2 CO.sub.3 as catalyst (4.9 g K.sub.2 CO.sub.3 per 192.2 g isophorone) and also using dimethylformamide. In contrast published Japanese application JP-A-116038 teaches the reaction of hydrogen cyanide in the presence of basic catalysts such as e.g. alkali-metal cyanides, -carbonates, -oxides, -hydroxides and -alcoholates, and glycols with isophorone. The glycols are added in an amount 1 to 50 times greater than the catalyst. High yields of isophorone nitrile are disclosed in the examples, but the reaction times using sodium and potassium carbonate or sodium cyanide are in the range of the methods already previously known at the time. The space-time yield thus continued to remain unsatisfactory. As a result of the use of glycols, the workup becomes more expensive and the economy is reduced.
Other methods which have been disclosed involve the use of quaternary ammonium or phosphonium hydroxides - cf. published Japanese application JP-A 61-33157 - or diazabicycloalkenes - cf. published Japanese application JPA 61-33158 - as catalyst. However, these catalysts are quite expensive.
German patent DE-PS 12 40 521 describes a method in which the reaction between isophorone and hydrogen cyanide takes place in the presence of alkaline catalysts placed on solid carriers at 50.degree. to 350 C. Preferably, alkali hydroxides or alkali cyanides are deposited on the carriers as catalysts. The preparation of the carrier catalysts is quite complicated and carrying-out the reaction is expensive, since the HCN concentration in the reaction mixture must be maintained low. For this purpose, the hydrocyanic acid has to be diluted in the gas phase with nitrogen. The method thus does not appear to be economical for industrial application in spite of the high yields indicated.
Finally, 1,3,3-trimethyl-5-oxo-cyclohexane carbonitrile can also be obtained from isophorone and alkali cyanides in an aqueous-organic two-phase system in the presence of a phase-transfer catalyst - see published European application EP B 028 179. However, this method requires the use of an alkali cyanide, which is more expensive than hydrogen cyanide as well as a not inconsiderable amount of an expensive transfer catalyst; the workup of the aqueous phase results in a considerable accumulation of salt and consequently problems of disposal.